Enable Team Tournaments, add Tournament Trees, implement temporary tree persistance (#66)

Co-authored-by: David Rodenkirchen <drodenkirchen@linetco.com>
Reviewed-on: #66

tournament_data/.gitignore

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+*.json

tournament_data/README.md

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+# Tournament data
+
+This directory contains JSON files for tournament trees.
+
+This is a temporary solution until the automatic tournament tree generation is completed.
+
+# Structure
+
+## Naming
+
+Tournament name with `_` as separators and `.json` suffix.
+
+## JSON structure
+
+```json
+{
+  "rounds": [
+    [
+      {
+        "opponent_1_id": 1,
+        "opponent_2_id": 2,
+        "winner": 1
+      },
+      {
+        "opponent_1_id": 3,
+        "opponent_2_id": 4,
+        "winner": null
+      }
+    ],
+    [
+      {
+        "opponent_1_id": 1,
+        "opponent_2_id": null,
+        "winner": null
+      }
+    ]
+  ]
+}
+```
+
+## ToDo
+
+- Make start button in UI generate initial `.json` file for started tournament